Diffusion pump



July 25.1961 A. A. LmFoRs 3,332,608

DIFFUSION PUMP Q Filed Jan. 24, 1966 TUBE CONNECTOR [:1

2 Sheets-Sheet 1 DIFFUSION) l 9 24 PUMP c x b 2 TIMER c g 28/ Q/{C //I T/ I 1/ E o W c I58 I 2 22 H- F IG.2

l l1 l3 m BA 6 July 25, 1967 "A. A. LANDFORS 3,332,608

DIFFUSION PUMP Filed Jan. 24, 1966 Sheets-Sheet 2 STEPS UNLOAD FINISHED TUBE 5 V ALOAD NEW TUBE JVALVE 24 CLOSED VALVE 26 OPEN FOR AIR RELEASE JVALVE 22 CLOSED 0 $0 6 0' e'o'lo TIME (SECONDS) PIC-3.3

TOP NOZZLE United States Patent 3,332,608 DIFFUSION PUMP Arthur A. Landfors, Sharon, Mass, assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Jan. 24, 1966, Ser. No. 522,523 Claims. (Cl. 230-101) The present invention relates to oil diffusion pumps and has particular application to oil diffusion pumps used in repetitive rapid cycle operations such as cathode ray tube aluminizing and utilizing chemically stable oils (such as silicone oils) which may be heated at atmospheric pressure.

In operations such as aluminizing, a series of pumping systems is provided. Each pumping system has a diffusion pump with an inlet and a discharge foreline connected to a mechanical backing pump. A valve is provided in the foreline. A tube to be aluminized is connected to the diffusion pump inlet. The mechanical pump rough pumps the tube through the diffusion pump and open foreline valve. Meanwhile the oil in the diffusion pump is heated. When the pressure in the diffusion pump is sufficiently low, the oil boils and is formed into jets which provide fine pumping to harden the vacuum for the aluminizing operation in the tube. After a predetermined time interval, the pumping system is air released, the aluminized tube is unloaded and a new tube is loaded.

In order to get maximum cycle speed, it is desirable that the diffusion pump shall have the capability to break its jets very quickly, prior to air release, and to reform the jets very quickly in the pumpdown phase of the next subsequent cycle.

The object of this invention is to provide an improved diffusion pump which meets the foregoing criteria with structural changes which involve very little extra cost compared to conventional pumps, yet offer reliable performance in meeting the foregoing criteria to provide substantial improvement in the time and quality of the cyclic processes and systems incorporating such improved pumps.

The invention is now described in detail with reference to the accompanying drawings wherein:

FIG. 1 is a diagrammatic view of a pumping system incorporating the improved diffusion pump of the invention, which is illustrated in simplified form;

FIG. 2 is a sectional view showing the diffusion pump as actually constructed and used in the working examples given herein;

FIG. 3 is a bar graph showing typical time sequences of operations when using the pump;

FIG. 4 is a partly sectional view of a second embodiment of the invention; and

FIG. 5 is a partly sectional view of a third embodiment of the invention.

Referring now to FIG. 1, the elements of each pumping system are the tube to be evacuated, a connector, a diffusion pump 19, a foreline with a foreline valve 22 and a mechanical backing pump 30. The diffusion pump (the lower portion of which is shown in the drawing in simplified form) comprises a pump body 11 with cooling coils 12. A pool of silicone pump oil 13 is contained in a reservoir at the bottom of the pump body and held at high temperature by a heater 14. A vapor jet nozzle assembly 15 is contained in the body.

Boiling oil vapors from the reservoir 15 rise up within the assembly 15 and are expanded through the nozzles 15A, 15B, 15C, etc. into the annular pumping space around assembly 15 to pump air. The oil vapors are collected on the cooled wall 11 as condensate and they drip down the wall to return to the reservoir. Holes 13A in 3,332,698- Patented July 25, 1967 the lower portion of the vapor jet assembly 15 provide a return path to the interior of the vapor jet assembly. A foreline trap (not shown) may be provided in the foreline 20 to limit oil carryover.

The pump is charged with a silicone oil such as DC704. In a four-inch diffusion pump, the charge of oil would be 300 00., preferably to provide an oil level between and inch. The advantage of such oils is that they can be held at operating temperature while the system is raised to atmospheric pressure. This allows rapid cycling, but unfortunately increases the dangers of oil loss. Frequent addition of oil to maintain an adequate level is undesirable as a matter of oil cost and is inconsistent with the needs of an automatic, low maintenance, aluminizing line. Aside from these considerations, migration of oil vapors back into the tube to be aluminized would be deleterious to the coating and substantial carryover of hot oil into the mechanical backing pump is deleterious to the backing pump and leads to objectionable mechanical pump exhaust fumes.

There are several approaches to limiting oil loss and/ or oil migration. First, the above-noted foreline trap limits oil carryover from the diffusion pump to the mechanical pump. But the reduction alone is insufficient when it is :desired to run several hundred cycles without maintenance. A second approach involves cutting off the pump heater to cool the pump oil so that it does not escape and this approach may be augmented by an internal quench coil in or adjacent the oil pool, as described in the patent to Gerow et al., 2,933,233. But the time required to heat and cool is still a significant obstacle to rapid cycle operation. Also heater lifetime is limited if the heater is shut down at the end of each cycle. Third, it is possibleto provide an inlet valve for the diffusion pump. But expense and added maintenance and reliability problems are disadvantages of this approach. Fourth, it is possible to provide for a sweep of air through the diffusion pump during the air release portion of each cycle. To accomplish this, a restricted bypass around the foreline valve or a small auxiliary pump is provided as described in the patent to Power 2,902,206 to provide continuous suction through the diffusion pump during air release so that oil vapors which would otherwise migrate to the diffusion pump inlet are instead routed to the foreline. But the auxiliary suction source is inherently limited in speed because the large volume of oil vapor tends to charge the auxiliary suction means with a mixture of air and oil which disturbs mechanical pump operation.

In the past I have developed (with Steinherz) a fifth approach to reducing oil loss. A bafiie is placed above the oil pool. Just before an air release (about 4 seconds before) water is admitted to the baffle to cool it. The jets of the pump are quickly broken by typical operating forepressures (50 microns and up). The oil pool temperature stays at its operating level while the baffle condenses vapors above the pool. When the pump is air released, boiling stops. Once baffle cooling is stopped, the diffusion pump can take hold again in a short time (in the next subsequent pumping cycle) against high forepressures (e.g. reforming the jets in 35 seconds against a forepressure of 200 microns). We have built such pumps and they have been used in industry to provide considerable reduction in cycle time and low oil loss.

The present invention involves a further improvement over the fifth approach described above affording a more complete suppression of oil loss. According to the principal feature of the present invention, an internal cooling coil of cylindrical spiral configuration is placed above the pump oil reservoir. An annular disc is placed around the coil about halfway up its length and a solid disc is placed above the center of the coiL This arrangement causes a majority of the vapors produced in the oil pump reservoir to make double transit of the coil (going into and out of the coil). When cooling water is admitted to the coil, the vapors in transit through the coil are condensed on the coil turns and dripped back into the oil pool. After air release, air coming into the pump enters the vapor jet assembly through the nozzle and as the air pressure in the assembly rises boiling stops. At the time of air release there is no significant oil vapor in transit within the pump. When the present invention is used, essentially the only oil vapor available for forming part of the mist is the small quantity of oil vapor within the cooling coil. However, DC704 has a vapor pressure of about 1 millimeter of mercury at 440 F the normal boiling temperature. Even though the atmospheric pressure on the sur face of the oil has suppressed boiling there will still be some evaporation which becomes mist. Now the oil loss from this source is of no concern per se. But to the extent that this remaining mist appears at the inlet it may interfere with the next aluminizing cycle. According to a further aspect of the present invention, I account for this remaining mist by adding a bypass line to the foreline, as described in the above-cited Power patent, or some other source of auxiliary suction on the foreline. The limited amount of mist produced in my pump is perfectly compatible with both the Power structure and with the maintenance of full operating temperature in the silicone oil. Thus, the combination produces an advantage(a) maintenance of peak oil temperature during the air release phase without significantly adverse migration or oil loss efiectsbeyond the advantages normally afforded by the separate parts of the combination, (b) limitation of oil loss by a condensing baffle within the vapor jet assembly (Steinherz-Landfors) and (c) limitation of migration by a foreline valve bypass (Power).

Referring to FIG. 1, the internal cooling coil is indicated at 16. Vapor flow through the coil, as defined by the bafiies 18 and 19, is indicated by the arrows X. Baffle 18 is a solid disc soldered to the top of the coil and battle 19 is an annular disc (a washer) soldered to the wall of the jet assembly 15. While the bafiie 18 inherently turns cold during cooling of the coil 16, the temperature of this bafiie is not critical to working of the invention. The cooling coil itself has the form of a cylindrical spiral which allows easy access to the bottom of the pump body for cleaning (after the jet assembly is lifted out through the pump inlet). It is preferred that the space between turns will be equal to about one to two pipe diameters. The total escape area between turns (above bafile 19) should be greater than the total escape area from nozzles 15A, 15B, 15C, etc. to prevent choking. The coil construction is simple and inexpensive to manufacture and it has the further advantage that it is free to expand and contract in response to thermal variations so that it is not stressed. The ends of the coil are brought out of the pump through the side wall via two seals (one of which is indicated at 17) located at the bottom of the pump below the oil level. The use of feedthroughs in the side wall of the pump avoids interference with the heater assembly.

The foreline valve 22 can be bypassed through a valve 24 (typically, inch opening compared to a one-inch opening for the foreline valve). An air release valve for the system is indicated at 26. A timer 28 controls operation of the above valves 22, 24, 26 and also controls operation of the cooling coil through a valve 32.

Referring now to FIG. 2, the lower end of the diffusion pump is shown in more detail. The boilerplate 6 has a finned construction, known per se. The operating oil levels are measured from the top of fins. The internal cooling coil has a portion 7 which extends below the top of oil pool 13 to complete the spiral over its full height for simplicity of construction and more stable support. The portion of the coil 16 passing through the oil pool 13 (feed and discharge runs and the spiral turns 7) have little efiect on oil temperature because of their limited surface area exposed to the oil and because the pump heater 14 operates continuously. The vapor jet assembly 15 and the pump body have annular dimples at 8 and 9, respectively, to limit backstreaming from the portion of the oil pool outside assembly 15. FIG. 3 shows a desirable air release phase for repetitive operating cycles for the timer 28. Starting from the time zero, the system is under vacuum and the aluminizing phase of the cycle is complete. Valve 32 is opened to admit water to coil 16. At the same time or within a few seconds thereafter the foreline valve 22 is closed. The jet is very quickly broken. At 15 seconds from zero, the timer opens the air release valve 26 (which quickly raises the system pressure to atmospheric). Five seconds after air release, or twenty seconds from zero, the bypass valve 24 is closed. Ten seconds later, or 30 seconds from zero, cooling is terminated by closing the valve 32. During the interval from 20 seconds to seconds, the operator may remove the tube and replace it with a new one for the next cycle. With both valves 22 and 24 closed, there is no air pressure dilferential across the tube to oppose lifting of the tube. At the end of 20 seconds, the next pumping cycle is started by closing air release valve 26 and opening valves 22, 24. I ran a four-inch pump system with a test dome in lieu of a tube through such a cyclic operation with a pumping interval of seconds for a total time per complete cycle of 300 seconds (120 air release phase, 180 pump down phase). The diffusion pump was charged with 300 cubic centimeters of DC704 oil. After 500 of such cycles, consecutively, I measured the oil loss. The total oil loss was 25 cubic centimeters; this is well below the loss which the pump can tolerate before maintenance is required.

I also ran a modified cyclic operation where no bypass line was used around the foreline valve. The modified cycle had the time sequence as follows:

Time zero: Admit water to coil 16; close foreline valve 22; air release valve 26 stays closed.

Zero plus 15 seconds: Open air release valve.

Zero plus 45 seconds: Cut off water to coil 16.

Zero plus 120 seconds: Close air release valve, open foreline valve.

Pumpdown from Zero plus 120 to zero plus 420 seconds. I

This modified cycle was run 550 times, consecutively, and resulted in a total oil loss of 25 cubic centimeters.

Referring now to FIG. 4, another embodiment of my invention is shown. This incorporates a conventional fractionating tube 151. In this embodiment, the disc 18 takes the form of a flange secured to the tube 151. Sufficient space is left between the inner diameter of the spiral coil and the outer wall of the fractionating tube to allow passage of oil vapors without choking. The coil going up the fractionating tube avoids regulation by coil 16, but oil loss and backstreaming from this source are so small that they do not have a significantly adverse effect.

Referring now to FIG. 5, an embodiment of my invention is shown for use in a pump having a conventional tie-down rod 152. The bathe 18 is held to the tie down rod by a pair of nuts 181 threaded on the rod. The construction is otherwise the same as FIG. 2.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as well as the embodiments described herein.

What is claimed is:

1. An improved oil diffusion pump for repetitive, rapid cycle operation, comprising in combination:

(a) a pump body consisting essentially of a vertically arranged tube with an oil reservoir at the lower end thereof and means for heating oil therein;

(b) a discharge foreline connected to the side of the pump body at the lower portion thereof, but above the oil reservoir;

(0) a vertically arranged tubular vapor jet assembly contained within the pump body, said assembly having a lower open end in the oil reservoir and at least one jet nozzle means therein disposed above the point of connection of the foreline to the body;

(d) an internal cooling coil having the form of a cylindrical spiral, the coil being disposed within the tubular vapor jet assembly below the nozzle means thereof, at least a portion of the coil being disposed above the oil reservoir;

(e) bafiie means comprising a first baffie element above the cooling coil and a second bafiie element beside the cooling coil, the baffie means being constructed and arranged to provide a double transit through the coil for a majority of upwardly migrating vapors from the oil reservoir; and

(f) means for selectively supplying coolant to the coil 2. The pump of claim 1 wherein said foreline (b) contains a foreline valve and further comprising:

(g) a bypass valve having an inlet connection to the foreline between the pump body and foreline valve, the bypass valve having a smaller opening than the foreline valve; and

(h) automatic control means connected to said means for supplying coolant, said bypass valve and said foreline valve to provide a rapid automatic sequence of operation thereof wherein coolant is admitted to the coil and the foreline valve closed essentially simultaneously, followed by closure of the bypass valve, followed in turn by cut off of the coolant supply.

3. The pump of claim 1 wherein the cooling coil extends downs into the pump oil reservoir and wherein said coolant supply means (e) includes coolant feed and discharge pipes connected to the ends of the coil and passing through the pump oil reservoir and through a side wall of the pump body.

4. The pump of claim 1 wherein the vapor jet assembly (0) further comprises a central fractionating tube extending down through the cooling coil (d), the parts being constructed to leave an annular space between the cooling coil and fractionating tube, the topbaffle element of said bafile means (e) forming a flange extension of the outer surface of the fractionating tube.

5. In combination, a diffusion pump connected to a mechanical pump via discharge foreline, a valve in said foreline, the diffusion pump having a vapor jet assembly, a heated oil reservoir therein, and bafile means within the vapor jet assembly above the oil reservoir with means for selectively cooling the bathe means to condense pump oil vapors, a bypass around the foreline and automatic control means connected to said cooling means and to said foreline valve for starting the flow of cooling water and shutting the foreline valve while maintaining the heating of the oil reservoir, whenever the diffusion pump is air released whereby the pump oil is maintained at operating temperature during air release, without significantly adverse backstreaming or oil loss effects.

References Cited UNITED STATES PATENTS 2,806,644 9/1957 Warren 230101 2,851,987 9/1958 Klinder 230-101 2,865,560 12/1958 Franceschini 230-101 2,887,618 5/1959 Reid 230- 2,899,127 8/1959 Power 230102 2,902,206 9/1959 Power 23045 2,933,233 4/1960 Gerow 230101 3,122,896 3/1964 Hickey 230101 DONLEY I. STOCKING, Primary Examiner.

W. I. KRAUSS, Assistant Examiner. 

1. AN IMPROVED OIL DIFFUSION PUMP FOR REPETITIVE, RAPID CYCLE OPERATION, COMPRISING IN COMBINATION: (A) A PUMP BODY CONSISTING ESSENTIALLY OF A VERTICALLY ARRANGED TUBE WITH AN OIL RESERVOIR AT THE LOWER END THEREOF AND MEANS FOR HEATING OIL THEREIN; (B) A DISCHARGE FORELINE CONNECTED TO THE SIDE OF THE PUMP BODY AT THE LOWER PORTION THEREOF, BUT ABOVE THE OIL RESERVOIR; (C) A VERTICALLY ARRANGED TUBULAR VAPOR JET ASSEMBLY CONTAINED WITHIN THE PUMP BODY, SAID ASSEMBLY HAVING A LOWER OPEN END IN THE OIL RESERVOIR AND AT LEAST ONE JET NOZZLE MEANS THEREIN DISPOSED ABOVE THE POINT OF CONNECTION OF THE FORELINE TO THE BODY; (D) AN INTERNAL COOLING COIL HAVING THE FORM OF A CYLINDRICAL SPIRAL, THE COIL BEING DISPOSED WITHIN THE TUBULAR VAPOR JET ASSEMBLY BELOW THE NOZZLE MEANS THEREOF, AT LEAST A PORTION OF THE COIL BEING DISPOSED ABOVE THE OIL RESERVOIR; (E) BAFFLE MEANS COMPRISING A FIRST BAFFLE ELEMENT ABOVE THE COOLING COIL AND A SECOND BAFFLE ELEMENT BESIDE THE COOLING COIL, THE BAFFLE MEANS BEING CONSTRUCTED AND ARRANGED TO PROVIDE A DOUBLE TRANSIT THROUGH THE COIL FOR A MAJORITY OF UPWARDLY MIGRATING VAPORS FROM THE OIL RESERVOIR; AND (F) MEANS FOR SELECTIVELY SUPPLYING COOLANT TO THE COIL (D). 